This invention relates generally to transaxles including a hydrostatic transmission (“HST”) commonly used with riding lawn mowers and similar small tractors. Such tractors generally use an engine having a vertical output shaft which is connected to the transaxle via a conventional belt and pulley system. A standard HST for such a transaxle includes a hydraulic pump, which is driven by the engine output shaft, and a hydraulic motor, both of which are usually mounted on a center section. Rotation of the pump by an input shaft creates an axial motion of the pump pistons. The oil pressure created by this axial motion is channelled via porting to the hydraulic motor, where it is received by the motor pistons, and the axial motion of these pistons against a thrust bearing causes the motor to rotate. The hydraulic motor in turn has an output shaft which drives the vehicle axles through differential gearing.
Among the advantages of transaxles with hydrostatic transmissions are the reduction of the number of parts and in the size of the unit, and, in some instances, the elimination of mechanical gears. As is known in the art, the use of a transaxle having a hydrostatic transmission enables the manufacturer to include all necessary elements in one unit, whereby the transaxle is easily incorporated into the tractor design, as it requires only the addition of a belt to connect it to the motor and a control lever for changing speed and direction. While the basic principles of transaxles with an HST are well known in the prior art, there are several disadvantages of present transaxles with HST designs. These disadvantages, and the present invention's means for overcoming them, are set forth herein.
A major problem with some prior transaxle designs is that the transmission is too large and too expensive to be used with the smaller tractors where it would be most effective. An attempt to solve this problem is shown in Okada, U.S. Pat. Nos. 4,914,907 and 4,932,209. The Okada '209 patent discloses a first mechanical deceleration means, namely the gear on the motor shaft and countershaft within the axle housing, and a second mechanical deceleration means in the differential. The gearing in the deceleration means eventually transmits power to the differential gears, which are then used to drive the output axle. However, these mechanical deceleration units add unnecessary weight and expense to the unit. An object of the present invention is to provide an transaxle design which does not require such additional mechanical deceleration means.
Another variation on the standard transaxle with HST design is shown in Thoma, U.S. Pat. No. 4,979,583. This patent teaches the segregation of the hydraulic units from the remaining portions of the transaxle through the use of separate segregated cavities to house each. In addition, the pump and motor in the Thoma design are mounted back-to-back, so that the input and output shafts have the same orientation. Thus additional gear units are required to re-orient the rotation of the output shaft so that it is parallel to the ultimate drive axle. Further gears then drive a differential which rotates the drive axle. This additional gearing adds weight to the unit and expense to the manufacturing process.
Thus, the Okada and Thoma designs present problems from the standpoint of manufacturing a small, economical transaxle including an HST which is easily adaptable to different size tractors or axle configuration. Okada requires multiple gearing and Thoma requires a housing having segregated cavities. The present invention is designed to overcome these and other problems in the prior art by providing a compact, economical transaxle with HST which substantially reduces the number of moving parts previously required.